Sintering

When fine powder is heated to high temperatures, the powder particles stick together and solidify even at temperatures below the melting point. This is an important phenomenon that is the basis of powder metallurgy and ceramics, and is used to make ceramics and various industrial materials. When sintering occurs, atoms move actively at the contact points between particles, passing through the surface and inside of the particles or sublimating, and as heating continues, atoms accumulate to fill the gaps between the particles, and what was initially sponge-like becomes a dense mass with few pores. Snow sticking together and solidifying even at temperatures below 0°C is also a type of sintering phenomenon. In addition, if the fine powder ore produced when crushing iron ore is put into a blast furnace as is, it will cause shelf hanging (a condition in which the ore gets caught in a part of the shaft and cannot descend), which will interfere with operation, so to avoid this, it is sintered to make a suitable size of a breathable mass that is difficult to crush.

The driving force behind sintering is mainly the surface tension of the material, which is the same type of force as the capillary pressure that lifts water in a thin glass tube. At the contact points between different types of powder particles, the particles try to mix and become homogeneous, and the forces that try to form compounds also cause the atoms to move, making the sintering phenomenon extremely complex. In addition, when sintering is performed under pressure, the contact points between the particles are compressed, the atomic movement becomes even more active, and even materials that are difficult to sinter can be solidified relatively easily. If some of the components melt during sintering and seep into the gaps between the powder particles, the particles are attracted to each other by the capillary pressure of the molten liquid, and a dense mass is formed in a short time. This is called liquid phase sintering, and is often used to sinter powders that are difficult to sinter as they are, such as metal powders and ceramic powders that have high melting points. In industry, the sintering temperature and time are changed depending on the type and properties of the powder and the intended use of the product. In addition, metal powders and other materials are difficult to sinter if there is an oxide on the particle surface, so they are sintered in a reducing gas such as hydrogen or decomposed ammonia gas, or in a vacuum.

[Ryuzo Watanabe]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

細かい粉末を高温に加熱したときに、融点以下の温度でも粉の粒子が互いに付着して固まること。粉末冶金(やきん)や窯業のもとになる重要な現象であり、陶磁器や種々の工業材料をつくるのに利用される。焼結がおきているときには粒子どうしの接触部で原子が粒子の表面や内部を通ったり昇華したりして活発に動いており、加熱を続けるうちに粒子間のすきまを埋めるように原子が堆積(たいせき)し、初めはスポンジ状を呈していたものも孔(あな)の少ない緻密(ちみつ)な塊になっていく。雪が0℃以下の温度でも付着して固まっていくのも一種の焼結現象である。また鉄鉱石を粉砕したときにできる細かい粉鉱石は、そのまま溶鉱炉に入れると棚吊(つ)り（鉱石がシャフトの局部にひっかかって下降しなくなる状態）などをおこし操業に支障をきたすので、それを避けるため焼結して適当な大きさのつぶれにくい通気性のある塊にする。

　焼結の原動力は主として物質の表面張力であるが、これは細いガラス管の中の水を持ち上げる毛管圧力と同種の力である。種類の異なる粉末粒の接触部では混じり合って均質になろうとしたり、化合物をつくろうとする力も原子の移動を引き起こし、焼結現象はきわめて複雑になる。また、圧力をかけながら焼結すると粒子間接触部は圧着され、原子の動きもさらに活発になり、焼結しにくい物質でも比較的容易に固められる。焼結中に成分の一部が溶けて粉末粒子のすきまにしみ込んでいく場合には、融液の毛管圧力により粒子は互いに引き付けられ、短時間で緻密な塊ができる。これは液相焼結とよばれ、融点の高い金属粉やセラミック粉のようにそのままでは焼き固めにくい粉末を焼結するときによく用いられる。工業的には、粉末の種類や性質および製品の使用目的に応じて焼結の温度、時間を変える。また、金属粉などは粒子表面に酸化物があると焼結しにくいので、水素や分解アンモニアガスのように還元力のあるガス中や真空中で焼結する。

［渡辺龍三］

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